This invention relates generally to voltage controlled oscillators and, more particularly, to push-push voltage controlled oscillators which operate in the microwave frequency spectrum.
Voltage controlled oscillators (VCO's) are utilized in a wide variety of electronic devices, for example, for voltage-to-frequency conversion and for RF signal generation. A voltage controlled oscillator generates a sinusoidal output signal which varies in frequency as a function of an input control voltage. One common type of voltage controlled oscillator includes an active feedback element, such as a transistor, and a frequency-determining element, such as a resonant tank circuit. The transistor provides positive feedback to the tank circuit, causing the tank circuit to oscillate at a resonant frequency f.sub.0 The resonant tank circuit typically includes a variable-capacitance element, such as a varactor diode, which allows the resonant frequency of the tank circuit to be easily varied by the input control voltage.
In many applications, such as in broadband receivers for microwave communications systems, microwave signals must be generated over a wide frequency range with low phase or side band noise. One type of broadband voltage controlled oscillator which generates an output signal with low phase noise is a frequency-doubling or push-push voltage controlled oscillator. A push-push VCO includes two identical voltage controlled oscillators connected in parallel and in phase opposition. The output signal of each oscillator contains fundamental and higher order harmonics of the oscillator's resonant frequency f.sub.0, but because the two oscillators are 180 degrees out of phase, the fundamental and odd harmonics cancel each other out and the even harmonics add together to produce an output signal at twice the resonant frequency f.sub.0. The output signal also includes the higher order even harmonics of the resonant frequency f.sub.0, but these components are generally negligible.
A typical push-push voltage controlled oscillator of the prior art, described in U.S. Pat. No. 4,527,130 to Lutteke, U.S. Pat. No. 3,939,429 to Lohn et al. and U.S. Pat. No. 4,360,790 to Heise, utilizes series LC resonant tank circuits. Although this type of push-push voltage controlled oscillator provides good phase noise characteristics, there is still need for further improvement. The present invention is directed to this end.